mem-ruby: add addressOffset util

mem-ruby: add addressOffset util

Returns the offset of an address with respect to a base address.
Looks unnecessary, but SLICC doesn't support casting and the '-'
operator for Addr types, so the alternative to this would be to add
more some helpers like 'addrToUint64' and 'uint64ToInt'.

Change-Id: I90480cec4c8b2e6bb9706f8b94ed33abe3c93e78
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31270
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>